The Pentagon is proposing to counter the resilient Taliban insurgency in Afghanistan by adding thousands more U.S. troops, putting them closer to combat and pounding militants with more airstrikes, according to a senior Defense official.

There are about 8,400 U.S. troops in Afghanistan, down from the peak of 100,000 in 2010-'11. The request for at least 3,000 more American combat forces to bolster battered Afghan security forces was signaled earlier this year when the top U.S. commander there, Army Gen. John Nicholson, told the Senate the security situation had deteriorated.

The military's proposal, which requires White House approval, would allow U.S. troops to partner with Afghan forces closer to the fight, said the Defense official who was not authorized to speak publicly about the plan. U.S. troops in Afghanistan have dual responsibilities: advising Afghan security forces and a counter-terrorism mission against al Qaeda and Islamic State terrorists.

Late last month, Defense Secretary Jim Mattis visited Kabul to hear directly from Nicholson. Mattis' visit came days after an attack by a Taliban-affiliated militant killed 140 Afghan troops, most of whom were unarmed in a mosque praying at their base.

Under the Obama administration, the Pentagon had been moving to end its presence in Afghanistan dating to 2001. Then, in the aftermath of the 9/11 terror attacks, U.S. forces helped topple the Taliban government that had given shelter to Osama bin Laden's al-Qaeda terrorist organization.

But pleas from U.S. commanders about the tenuous security situation prompted President Barack Obama to reverse course and leave the current force in place. However, there have been restrictions on how close Americans could accompany Afghan forces in combat and on bombing Taliban targets. Those rules were eased last year, and the Pentagon's proposal would grant added authority for air strikes.

Mattis also learned on his trip from U.S. intelligence officials that the Taliban have been receiving increased munitions shioments, including machine guns, and funding from Russia. Nicholson also acknowledged reports of Russian assistance to the Taliban.

An added complication in Afghanistan is the presence of Islamic State fighters, known as ISIS-Khorasan, or ISIS-K. Nicholson attacked the insurgents with one of the largest conventional munitions in the U.S. arsenal, the Massive Ordnance Air Blast, or MOAB, also referred to as the Mother of All Bombs.

The U.S.-led coalition has killed more 538 Islamic State fighters in Afghanistan since March. The strikes have reduced ISIS strength from about 3,000 militants in 2015 to about 800 today, according to the military official.

The fight with ISIS-K claimed the lives of two U.S. Army Rangers late last month.